ensure /data/tombstones exists on all Android devices

/data/tombstones is referenced by core platform code, but is not
guaranteed to exist on all Android devices. Move the directory
creation out of device specific files and into the core
init.rc file.

Bug: https://code.google.com/p/android/issues/detail?id=93207
Change-Id: I94ae5199a6a32c4fe555ca994fc4a8345e0c9690

rootdir/init.rc

@@ -290,6 +290,7 @@ on post-fs-data
     mkdir /data/app-lib 0771 system system
     mkdir /data/app 0771 system system
     mkdir /data/property 0700 root root
+    mkdir /data/tombstones 0771 system system
 
     # create dalvik-cache, so as to enforce our permissions
     mkdir /data/dalvik-cache 0771 root root